The stratum corneum from animal skin arises as a result of a programmed differentiation of underlying epidermal keratinocytes. The terminal events involved in the formation of this non-living structure from living stratum granulosum cells are complex and the molecular mechanisms are incompletely understood. The process involves nuclear expulsion, externalization of lipid-containing lamellar bodies, enzyme-catalyzed degradation and selective loss of all major classes of macromolecules and cellular dehydration. The dehydrated cells become flattened and they adhere tightly to form the final laminated structure which provides a selective barrier to the entry or exit of substances to or from the skin.
Numerous lines of investigation have led to the principle that the extent of hydration of the stratum corneum is responsible for the clinical feel and appearance of dry skin. The stratum corneum from normal skin contains 10-20% water, whereas the stratum corneum from clinically dry skin contains 5-10% water. Dry stratum corneum can be treated by direct addition of water, by occlusion to prevent water evaporation or by addition of substances with humectant (i.e. water-retaining) activity. Humectants commonly used in skin moisturizing products include glycerol, urea, propylene glycol, mineral oil and pyrollidine carboxyllic acid. More recently, naturally-occurring macromolecules such as collagen, hyaluronic acid, elastin or placental proteins have been used as humectants. Although all these substances are reasonably good humectants, it is clear that none of them are involved in the natural mechanisms of stratum corneum moisturization.
Since first proposed in 1952 that the clinical feel and appearance of dry skin directly results from a lack of water in the stratum corneum, the skin care cosmetic industry has formulated creams and lotions designed to hydrate the structure. This is accomplished by addition of water to the formula, inclusion of film-forming occlusive oils and waxes which cause water accumulation by slowing its exit from the skin, or by addition of hygroscopic humectant substances which hold water to the surface. Numerous investigators have employed gravimetric techniques to measure the water-binding capacity of humectant substances, either alone or on isolated stratum corneum. Generally, the substance is equilibrated at 0% relative humidity (RH) to obtain the dry weight and then after re-equilibration at a given RH and temperature for 20 to 40 days, the substance is again weighed. This approach yields accurate information, but it is extremely time-consuming and requires large amounts of assay materials.
We have now discovered an extremely rapid and sensitive method to measure the water-binding capacity of substances. The method is an in vitro radioactive assay system . Assay results can be obtained within a 20-hour time period using microgram quantities of materials and the procedure can assess humectant properties of single ingredients, finished formulas, complex mixtures or biological structures such as while skin, stratum corneum or hair. Using standard humectant substances, our assay results for maximal water binding agree with published results which use the conventional time consuming and laborious gravimetric techniques described above.